1. Field of the Invention
The present invention generally relates to a method of controlling communications paths, and more particularly to a method for acquiring one or a plurality of communications paths from among a predetermined number of communications paths. Further, the present invention is concerned with a communication device carrying out the above method.
2. Description of the Prior Art
A computer system is known in which a plurality of computers, such as large-scale, general purpose computers, are connected together. These computers share execution of works or application programs.
FIG. 1 shows such a decentralized computer system having two computers #1 and #2, which are connected to each other via a communication medium 21. As shown in FIG. 1, the communication medium 21 has three communications paths 1, 2 and 3, which are multiplexed in a conventional multiplexing method. In the case shown in FIG. 1, computer #1 executes works A, B and C, and computer #2 executes works X, Y and Z. The works executed by computers #1 and #2 share the communication paths 1, 2 and 3 and communicate with each other, so that computers #1 and #2 proceed with execution of the works. More specifically, the works A, B and C executed by computer #1 use the communication paths 2, 1 and 3, respectively, and communicate with the shared works Y, X and Z. For example, computer #1 manages a group of the three communication paths 1, 2 and 3 provided by the communication medium 21.
A limited number of communication paths is available in one communication medium 21 in terms of the characteristic of the communication medium 21 and the performance of the CPU and memories.
FIGS. 2A, 2B and 2C show a procedure for acquiring communication paths. Referring to FIG. 2A, the computer C1 executes words 1 through n, which use communication paths 1 through n, respectively. The computer C1 also executes words 1 through n, so that the works 1 through n are shared by the computers C1 and C2. The maximum number of communication paths available in the communication medium 21 is equal to n, while each of the computers C1 and C2 has executable works 1 through n+1.
Referring to FIG. 2B, an emergency event has occurred in the computer C1, which has a request to send an emergency message to the computer C2. However, the computer C1 cannot send such an emergency message to the computer C2 via the communication medium 21 because the works 1 through n use the communications paths 1 through n, respectively. In other words, all the communication paths 1 through n have been allotted to the works 1 through n, and it is impossible to allot a communication path to the emergency event.
Referring to FIG. 2C, the work n is completed and the communication path n is released. The work n+1 which requests to send the emergency message to the computer C2 acquires the communication path n. Then, the work n+1 sends the emergency message to the computer C2 via the communication path n, so that the work n+1 executed in the computer C2 is informed of the occurrence of the emergency event.
As described above, in the case where all the communication paths are busy, a new work cannot be executed irrespective of whether or not the above new work is emergency until one of the works is completed and one of the communication paths is released.
However, it is required to send an emergency message to the remote computer without delay of time if an emergency event has occurred. For the above requirement, it is desirable to allot a communication path to an emergency event immediately after an emergency event has occurred.